A very substantial commercial demand exists for extruded plastic sheets. The plastic sheets may be used for protective wraps, for bags, or for coatings applied to substrates such as paperboard or wood. Plastic sheets often must meet specifications defining gas permeability, water permeability, strength, flexibility, light transmissivity, photo-degradability and reception to various adhesives and ink.
A unitary extruded sheet of a single polymer often will be unable to meet all of the various requirements for the intended end use of the sheet. As a result, laminated plastic sheets are widely employed, with the various laminated layers being formed from different polymers. Each layer may be extruded from a polymer with a specified characteristic in accordance with the requirements of the finished product. Thus, one laminated plastic layer may be selected for its strength, another layer may be selected as being substantially impermeable to gas, while another layer may be selected for good reception of adhesive and/or ink.
A sample specified laminated plastic sheet is depicted in FIG. 1 and is identified by the numeral 10. The laminated plastic sheet 10 is formed by three separate plastic layers 12, 14 and 16. The particular polymer defining each layer 12-16 is selected in view of characteristics attributable to that polymer. Additionally, the respective thicknesses of each layer 12-16 are specified in accordance with those characteristics. For example, a first layer 12 of a specified thickness "a" may be selected for its strength. A layer 12 with a thickness substantially less then "a" may be too weak, while a layer 12 with a thickness substantially greater then "a" may be too rigid for the specified end use. Similarly, the layer 14 may be specified to have a thickness "b" to achieve a required oxygen impermeability. A layer 14 with a thickness less then "b" may transmit too much oxygen therethrough, while a layer 14 with a thickness greater then "b" may be unnecessarily wasteful of the plastic and may affect flexibility of the laminated sheet 10. Similarly, the polymer of layer 16 may be selected for receptivity of ink and adhesives. The specified thickness "c" may be required to prevent transmission of ink or adhesives into adjacent layer 14 or 12. An actual specified laminated plastic sheet 10 may have many more layers than the three of the sample sheet 10 depicted in FIG. 1.
The laminated plastic sheet 10 may be manufactured with an extruder having a specially manufactured manifold into which various streams of molten polymer are fed. The manifold will be configured to spread and laminate the various polymer layers to define a laminated sheet 10 of approximately the specified dimensions. Special purpose manifolds can be very expensive to manufacture and offer little flexibility from one laminated sheet construction to another.
A more versatile manufacturing approach is to employ a feedblock in the extrusion apparatus. A prior art extrusion apparatus of this general type is depicted in FIGS. 2-4 and is identified generally by the numeral 18. The prior art extrusion apparatus 18 includes a plurality of entry ports 22, 24 and 26 which extend into a feedblock 28 for an initial extrusion of the polymer layers. The feedblock 28 communicates with a combining chamber 30 which thins and spreads the laminated layers prior to a die port 31. The prior art feedblock 28 comprises diverter plates 32, 34, 36 and 38 intermediate the entry ports 22-26 and the combining chamber 30. The diverter plates 32-38 are secured in face-to-face relationship with one another by bolts 40 and are machined to define rectangular lamella channels 42, 44 and 46 (each of uniform height) for receiving melt streams of polymers from the respective feed ports 22-26. Each prior art lamella channel 42-46 will define a depth selected as a function of the specified thickness "a", "b" or "c" of the respective layers 12-16 of the sheet 10 shown in FIG. 1.
Although feedblocks are widely employed, it is extremely difficult to achieve the specified configuration of the laminated plastic sheet 10 as shown in FIG. In this regard, it is necessary to closely match polymer viscosity in an effort to obtain uniform layer thicknesses across the width of the laminated plastic sheet 10. It has been found, however, that the laminated sheet produced from the prior art feedblock apparatus 28 will not produce the specified sheet configuration 10 depicted in FIG. 1, but rather resembles the sheet 10.sub.1 depicted in FIG. 5. In particular, the layers 12.sub.1, 14.sub.1 and 16.sub.1 of the sheet 10.sub.1 which are applied to a paperboard substrate 17 will be of varying thickness across the respective widths, including, for example, major thicknesses (above average thickness) adjacent opposed side edges of each layer and minor thicknesses (less than average thickness) at locations spaced inwardly from the side edges. One or more additional areas of major thicknesses of the layers 12.sub.1, 14.sub.1 and 16.sub.1 may further exist at more central regions of the sheet 10.sub.1. These variations in thickness will remain despite any down stream thinning that may occur in the prior art apparatus 10. As noted above, variations in the thickness of a plastic layer from the specified thickness can significantly alter the performance of the laminated sheet for its intended end use. For example, it may be difficult to subsequently adhere the laminated sheet materials 10.sub.1 to a substrate in view of the non-planar condition of opposed surfaces of the sheet. The sheet 10.sub.1 may be insufficiently flexible, too weak in selected locations, insufficiently impermeable to gas, or may exhibit a host of other problems at least locally thereon. Prior art attempts to deal with the problems depicted schematically in FIG. 5 primarily have been directed toward controlling the viscosity of the melt streams feeding into and/or passing through the extruder. These prior art attempts have been difficult to control and often have been ineffective.
In view of the above, it is an object of the subject invention to provide a feedblock assembly for a laminated plastic sheet with layers of uniform specified thicknesses.
It is another object of the subject invention to provide a feedblock assembly that can be readily modified to achieve a specified laminated plastic sheet construction.